This manuscript presents an experimental effort to directly measure contact areas and the details behind these scaled experiments on a randomly rough model surface used in the “Contact Mechanics Challenge” (2017). For these experiments, the randomly rough surface model was scaled up by a factor of 1000× to give a 100 mm square sample that was 3D printed from opaque polymethylmethacrylate (PMMA). This sample was loaded against various optically smooth and transparent samples of PDMS that were approximately 15 mm thick and had a range in elastic modulus from 14 kPa to 2.1 MPa. During loading, a digital camera recorded contact locations by imaging the scattering of light that occurs off of the PMMA rough surface when it was in contact with the PDMS substrate. This method of illuminating contact areas is called frustrated total internal reflection and is performed by creating a condition of total internal reflection within the unperturbed PDMS samples. Contact or deformation of the surface results in light being diffusely transmitted from the PDMS and detected by the camera. For these experiments, a range of reduced pressure (nominal pressure/elastic modulus) from below 0.001 to over 1.0 was examined, and the resulting relative contact area (real area of contact/apparent area of contact) was found to increase from below 0.1% to over 60% at the highest pressures. The experimental uncertainties associated with experiments are discussed, and the results are compared to the numerical results from the simulation solution to the “Contact Mechanics Challenge.” The simulation results and experimental results of the relative contact areas as a function of reduced pressure are in agreement (within experimental uncertainties).

Measurements of surface deformations as part of the “Contact Mechanics Challenge” were collected using digital image correlation (DIC). For these experiments, a scaled version (1000×) of the periodic and random roughness surface provided for the “Contact Mechanics Challenge” was used. A 100 mm × 100 mm scale replica of the surface, approximately 10 mm thick, was 3D-printed using an opaque polymethylmethacrylate and pressed into contact against flat, transparent polydimethylsiloxane (PDMS) sheets with dead weight loads. Four different formulations of PDMS were used, and the resulting elastic moduli ranged from 64 kPa to 2.1 MPa. The DIC technique was used in situ to measure the deformation of the PDMS surface at each load increment from 22.5 to 450 N. Surface deformations in and out of contact were measured across the entire apparent area of contact and overlaid with the measurements of contact area to provide a complete description of the surface profile during loading. A direct comparison between these experiments and the simulations regarding the gap within the contact at a reduced pressure of 0.164 agrees to within ±10% when normalized to the maximum gap. 

Atomic force microscopy (AFM) reveals that tribotronic control of friction using an external potential applied to a gold surface is possible for ionic liquid (IL) concentrations as low as 5 mol% in hexadecane. The IL used is trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate, in which both the cation and anion have surfactant-like structures, and is miscible with hexadecane in all proportions. For IL concentrations less than 5 mol% friction does not vary with applied potential, but for 5 mol% and above changing the potential changes the composition of the IL boundary layer from cation-enriched (negative potentials) to anion-enriched (positive potentials). As the lubricities of the cation-rich and anion-rich boundary layers differ, this enables active control of friction in oil-based lubricants.

One of the main parameters affecting finger friction, friction-induced vibrations in the finger, and consequently tactility is surface topography. Recently Skedung et al. performed finger friction measurements on fine controlled surfaces. These surfaces were sinusoidal with wavelengths from 0.27 to 8.8 μm and amplitudes from 0.007 to 6 μm. Building on those tests an analytical model for the contact was developed to explain the differences in friction coefficient. The contact was modelled as trapezoids in a circular pattern pressed against a sinusoidal plane. Results showed that the calculated contact area and therefore friction coefficient corresponded well with the measurements. This model can be used to see how the different surface parameters influence friction.

The wear and friction behavior of ultralow wear polytetrafluoroethylene (PTFE)/αalumina composites first described by Burris and Sawyer in 2006 has been studied intensively in the years hence. The mechanisms responsible for the remarkable improvement in wear over unfilled PTFE are not yet fully understood. The formation of tribofilms on the countersurface and the running face of the polymer is crucial to the ultra-low wear behavior of the composite on a metal countersurface. The complete chemical mechanism of transfer film formation and adhesion, and its role in the exceptional wear performance has yet to be elucidated. Some debate exists regarding the role of chemical interactions between the PTFE, the filler, and the metal countersurface. Some have concluded that chemical changes are not an important part of the ultralow wear mechanism in these materials at all. A “stripe” test allowed comprehensive spectroscopic studies of PTFE/α-alumina transfer films in various stages throughout development and led to a proposed mechanism which details the initiation and adhesion of the tribofilms formed on both surfaces of the wear pair. PTFE chains (carbon-carbon bonds) are broken mechanically during sliding and undergo a cascade of reactions to produce carboxylate chain ends that chelate to the metal surface and to the surface of e porous, friable alumina filler particles. This tribochemical process forms a robust polymeron-polymer system that protects the steel countersurface from abrasion, and the polymer surface from wear. The system is able to withstand hundreds of thousands, and possibly millions of cycles of sliding with almost no wear of the polymer composite after an initial period of high wear during run-in. A mathematical model in support of the hypothesis of mechanical scission of carboncarbon bonds in the backbone of PTFE in simple sliding contact is detailed, using the Hamaker model for van der Waals interactions between polymer fibrils and the countersurface (a cylinder and a flat surface). The proven necessity of ambient moisture and oxygen is explained in the mechanism, and model experiments using small molecules further support the assignment of reactions in the proposed mechanism to the processes at the sliding interface.

In this manuscript, a simple binary model is devised that describes the wear behavior of two blocks coupled under a constant, dynamically partitioned normal load. In this simple system, the frictional force is reacted by two independent springs and the blocks are allowed to move and wear independently based on system dynamics and kinematics. The only coupling between the blocks occurs through the partitioning of the applied normal load, which uses a pair of springs in parallel to model elasticity. This system is found to preferentially wear one of the blocks until two disparately unique conditions of steady wear are reached in the system: (1) a condition in which the partitioning of the load between the blocks yields equal wear and thus steady partitioning of the load and (2) a condition in which the pair of blocks go to zero wear by having one block not sliding but carrying all of the load and the other block completely slipping but carrying none of the load. These “Janus blocks,” the simplest of binary spring–block systems, begin life in a nominally identical state and then their behavior bifurcates, producing runaway or irregular wear. The onset of this instability can initiate from any differences in load partitioning, spring constants, friction coefficient, or wear rates (no matter how small). 

Improving the tactile aesthetics of products that can be described as touch intensive is an increasing priority within many sectors, including the furniture industry. Understanding which physical characteristics contribute to the haptic experience of a surface, and how, is therefore highly topical. It has earlier been shown that both friction and topography affect tactile perception. Thus, two series of stimuli have been produced using standard coating techniques, with systematic variation in (physical) friction and roughness properties. This was achieved through appropriate selection of matting agents and resins. The stimuli sets were then evaluated perceptually to determine the extent to which discrimination between pairs of surfaces followed the systematic materials variation. In addition to investigating the role of the physical properties in discrimination of the surfaces, their influence on perceived pleasantness and naturalness was also studied. The results indicate that changes in tactile perception can be understood in terms of friction and roughness, and that varying the matting agents (topography) and resins (material properties) in the coatings provide the controlling factors for furniture applications. Perceived pleasantness is associated with low friction and smoother topography, whilst perceived naturalness is found to be described by an interaction between tactile friction and the average maximum peak height of the surface features. Graphic Abstract: [Figure not available: see fulltext.] © 2021, The Author(s).

PTFE/α-alumina composites are well known to exhibit very low wear rates compared to unfilled PTFE and various other PTFE-matrix composites. The improved wear life of these composites is attributed in part to the formation of a uniform protective transfer film on the metal countersurface. It is postulated that the retention of transferred material and the recirculation of third bodies between the transfer film and running surface of the polymer composite are necessary for the maintenance of low wear within this tribological system. The accumulation of these third bodies was observed in reciprocating sliding tests on countersamples prescribed with aligned roughness. Wear performance of the polymer composite was tested as a function of the between the sliding direction and the aligned roughness of the countersample, ranging from parallel to perpendicular to the sliding direction. The wear rate of roughness oriented with the sliding direction was 300 times higher than roughness perpendicular to the sliding direction, revealing the importance of surface morphology and third body retention.

The wear and friction behavior of ultralow wear polytetrafluoroethylene (PTFE)/α-alumina composites first described by Burris and Sawyer in 2006 has been heavily studied, but the mechanisms responsible for the 4 orders of magnitude improvement in wear over unfilled PTFE are still not fully understood. It has been shown that the formation of a polymeric transfer film is crucial to achieving ultralow wear on a metal countersurface. However, the detailed chemical mechanism of transfer film formation and its role in the exceptional wear performance has yet to be described. There has been much debate about the role of chemical interactions between the PTFE, the filler, and the metal countersurface, and some researchers have even concluded that chemical changes are not an important part of the ultralow wear mechanism in these materials. Here, a "stripe" test allowed detailed spectroscopic studies of PTFE/α-alumina transfer films in various stages of development, which led to a proposed mechanism which accounts for the creation of chemically distinct films formed on both surfaces of the wear couple. PTFE chains are broken during sliding and undergo a series of reactions to produce carboxylate chain ends, which have been shown to chelate to both the metal surface and to the surface of the alumina filler particles. These tribochemical reactions form a robust polymer-on-polymer system that protects the steel countersurface and is able to withstand hundreds of thousands of cycles of sliding with almost no wear of the polymer composite after the initial run-in period. The mechanical scission of carbon-carbon bonds in the backbone of PTFE under conditions of sliding contact is supported mathematically using the Hamaker model for van der Waals interactions between polymer fibrils and the countersurface. The necessity for ambient moisture and oxygen is explained, and model experiments using small molecules confirm the reactions in the proposed mechanism. .

Cobalt chromium molybdenum alloys (CoCrMo) are commonly used as articulating components in joint prostheses. In this tribocorrosive environment, wear debris and metal ionic species are released and interact with proteins, possibly resulting in protein aggregation. This study aimed to investigate whether this could have an effect on the friction coefficient in a typical material couple, namely CoCrMo-on-polyethylene. It was confirmed that both Co(II) and Cr(III) ions, and their combination, at concentrations relevant for the metal release situation, resulted in protein aggregation and its concomitant precipitation, which increased the friction coefficient. Future studies should identify the clinical importance of these findings.

Atomic force microscopy has been used to study the effect of temperature on normal forces and friction for the room temperature ionic liquid (IL) ethylammonium nitrate (EAN), confined between mica and a silica colloid probe at 25 °C, 50 °C, and 80 °C. Force curves revealed a strong fluid dynamic influence at room temperature, which was greatly reduced at elevated temperatures due to the reduced liquid viscosity. A fluid dynamic analysis reveals that bulk viscosity is manifested at large separation but that EAN displays a nonzero slip, indicating a region of different viscosity near the surface. At high temperatures, the reduction in fluid dynamic force reveals step-like force curves, similar to those found at room temperature using much lower scan rates. The ionic liquid boundary layer remains adsorbed to the solid surface even at high temperature, which provides a mechanism for lubrication when fluid dynamic lubrication is strongly reduced. The friction data reveals a decrease in absolute friction force with increasing temperature, which is associated with increased thermal motion and reduced viscosity of the near surface layers but, consistent with the normal force data, boundary layer lubrication was unaffected. The implications for ILs as lubricants are discussed in terms of the behaviour of this well characterised system.

Soda lime silicate is an important industrial glass type, it is used in, e.g., windows, containers, household glasses, displays, cover glasses and in automotive glazing. The importance of soda lime silicate glass originates from the forming ability that enables low-cost manufacturing but also relatively high hardness, good chemical durability, and the essential transparency in the visible range. However, soda lime silicate glass suffers from brittleness and has a relatively low resistance towards surface defects. Therefore, is the practical strength of commercially available glass in the soda lime silicate glass family limited. The mechanical properties of glass is of great importance in various applications. Chemical strengthening (CS) of glass has become a successful process that today is used in many applications. It makes glass stronger by exchanging larger ions from a molten salt bath with smaller ions from the glass and thereby inducing compressive stresses in the surface. However, soda lime silicate glass is typically not well-suited for this process and therefore has the effect of dopants on surface mechanical properties as result of CS been studied. Hardness, reduced elastic modulus and crack resistance were studied. The CS was performed in a molten KNO3 salt bath at 450 °C. The results will be discussed in relation to the compositional and structural changes.

The present paper describes a method for non-destructive testing of the glass strength. Square 10 × 10 cm2 samples of annealed float glass was inflicted with a controlled defect in the centre of the atmospheric side using Vickers microindentation-induced cracking with a force of 2 N, 5 N and 10 N and compared to an un-indented reference. The samples were non-destructively tested using a nonlinear acoustic wave method resulting in defect values. The average of the defect values was found to linearly correlate to the indentation force in a log–log relationship. The samples were subsequently tested in a ring-on-ring setup that allows for an equibiaxial stress state. The indentation-induced cracking gave practically realistic strength values in the range of 45 to 110 MPa. The individual sample values for failure stress as a function of normalized defect value show linear trends with approximately half of the data within 95% confidence limit. In summary, this study provides an initial proof-of-concept for a non-destructive testing of the strength of glass.

Polytetrafluoroethylene (PTFE) filled with certain alumina additives has wear rates over four orders of magnitude lower than unfilled PTFE. The mechanisms for this wear reduction have remained a mystery. In this work, we use a combination of techniques to show that porous, nanostructured alumina microfillers (not nanofillers) are critical for this wear reduction. The microscale alumina particles break during sliding into nanoscale fragments. X-ray microtomography, transmission electron microscopy and infrared spectroscopy reveal nanoscale alumina fragments accumulated in the tribofilms. Tribochemically generated carboxylate endgroups bond to metal species in the transfer film and to alumina fragments in the surface of the polymer composite. These mechanically reinforced tribofilms create robust sliding surfaces and lead to a dramatic reduction in wear. © 2015 The Authors. 

The efficacy of ionic liquids (ILs) as lubricant additives to a model base oil has been probed at the nanoscale and macroscale as a function of IL concentration using the same materials. Silica surfaces lubricated with mixtures of the IL trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate and hexadecane are probed using atomic force microscopy (AFM) (nanoscale) and ball-on-disc tribometer (macroscale). At both length scales the pure IL is a much more effective lubricant than hexadecane. At the nanoscale, 2.0 mol% IL (and above) in hexadecane lubricates the silica as well as the pure IL due to the formation of a robust IL boundary layer that separates the sliding surfaces. At the macroscale the lubrication is highly load dependent; at low loads all the mixtures lubricate as effectively as the pure IL, whereas at higher loads rather high concentrations are required to provide IL like lubrication. Wear is also pronounced at high loads, for all cases except the pure IL, and a tribofilm is formed. Together, the nano- and macroscales results reveal that the IL is an effective lubricant additive - it reduces friction - in both the boundary regime at the nanoscale and mixed regime at the macroscale.

The lubrication of titania surfaces using a series of ionic liquid (IL)-hexadecane mixtures has been probed using nanoscale atomic force microscopy (AFM) and macroscale ball-on-disk tribometer measurements. The IL investigated is trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate, which is miscible with hexadecane in all proportions. At both length scales, the pure IL is a much more effective lubricant than pure hexadecane. At low loads, which are comparable to common industrial applications, the pure IL reduces the friction by 80% compared to pure hexadecane; while the IL-hexadecane mixtures lubricate the titania surface as effectively as the pure IL and wear decreases with increasing IL concentration. At high test loads the adsorbed ion boundary layer is displaced leading to surface contact and high friction, and wear is pronounced for all IL concentrations. Nonetheless, the IL performs better than a traditional zinc-dialkyl-dithophosphate (ZDDP) antiwear additive at the same concentration.

The “Contact Mechanics Challenge” posed to the tribology community by Müser and Dapp in 2015 detailed a 100 µm × 100 µm randomly rough surface with a root-mean-square gradient of unity, g ¯ = 1. Many surfaces, both natural and synthetic, can be described as randomly rough, but rarely with a root-mean-square gradient as steep as g ¯ = 1. The selection of such a challenging surface parameter was intentional, but potentially limiting for broad comparisons across existing models and theories which may be limited by small-slope approximations. In this manuscript, the root-mean-square gradients (g ¯) of the “Contact Mechanics Challenge” surface were produced on 1000 × scaled models such that there were three different surfaces for study with g¯=0.2,0.5, and 1. In situ measurements of the real area of contact and contact area distributions were performed using frustrated total internal reflectance along with surface deformation measurements performed using digital image correlation. These optical in situ experiments used the scaled 3D-printed rough surfaces that were loaded into contact with smooth, flat, and elastic samples that were made from unfilled PDMS: (10:1) E* = 2.1 MPa Δγ = 4 mJ/m2; (20:1) E* = 0.75 MPa Δγ = 3 mJ/m2; (30:1) E* = 0.24 MPa Δγ = 2 mJ/m2. All of the loading was performed using a uniaxial load frame under force control. A Green’s function molecular dynamics simulation assuming the small-slope approximation was compared to all experimental data. These measurements reveal that decreasing root-mean-square gradient noticeably increases real area of contact area under conditions of “equal” applied load, but variations in the root-mean-square gradient did not significantly alter the contact patch geometry under conditions of nearly equal real area of contact. Including g ¯ in the reduced pressure (p= P/ (E∗ g ¯)) reduced the root-mean-square error between the simulation (g ¯ = 1) and all experimental data for the relative area of contact as a function of reduced pressure over the entire range of surfaces, materials, and loads tested.

This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one..

Surface icing almost invariably derives from the precursory step of liquid water encountering the surface. Thus, slippery liquid infused porous surfaces, SLIPS, must possess steady wetting durability, and lubricant stability to function as a reliable hydro−/icephobic surface design especially in outdoor applications. Additionally, they should maintain their phobic performance under shear forces, and possess low sliding friction to act as a slippery, multirepellent surfaces. These characteristics are needed in variable applications ranging from moving and rotating blades to steady surfaces, operating in altering climate conditions. More profound durability testing is needed to examine the loss of surface functionality when the lubricant is depleted from the structure via various routes. In addition, the durability tests should be designed to serve the application-related purposes and thus, to reveal performance differences between slippery surfaces for further analysis and targeted end-use development. Here, we tested the wetting durability and stability of SLIPS with multicycle Wilhelmy plate by dipping the surfaces multiple times in water bath. Additionally, we examined the effects of centrifugal and friction-based shear stress to investigate the lubricant depletion from the structure. Tests that measure the durability and the stability of SLIPS designs are in great need in further developing functional slippery surfaces for real outdoor application coatings which encounter environmental stresses, e.g., wetting and icing. Acknowledging the material differences under specific stresses will guide designing the slippery surfaces towards more specific and functionable end-use applications

Polytetrafluoroethylene (PTFE) is widely regarded as an excellent candidate for solid lubrication in vacuum. However, it is often precluded from many practical applications due to its intrinsically high wear rate. Over the past decade, it has been discovered that small loading fractions of alumina nanofillers can increase the wear resistance of PTFE by three to four orders of magnitude. This dramatic increase in wear resistance has in turn prompted numerous tribological studies to examine the robustness of this performance. In this study, the wear and friction behavior of unfilled PTFE and PTFE and alumina nanocomposites were evaluated under a broad range of vacuum environments from 760 to 4 9 10-6 Torr. The nanocomposites of PTFE/alumina showed a dramatic increase in wear of over two orders of magnitude at the highest vacuum conditions. There appears to be an optimal vacuum environment around 1-10 Torr, in which these samples achieved the lowest wear rates of approximately 2.5 9 10-7 mm3/(Nm)

Over the last decade, researchers have explored an intriguing polymer composite composed of granular polytetrafluoroethylene (PTFE) 7C and alumina particles. This material is extraordinary because a very small amount of alumina additive (<5 wt%) decreased the wear rate of the PTFE composite by over four orders of magnitude. Previous studies have shown that this wear resistance was initiated and maintained by the formation of a stable, robust, and uniform polymeric transfer film on the surface of the countersample. Although its importance to this tribological system is clear, the transfer film itself has not been well understood. Careful spectroscopic analysis throughout the stages of transfer film development revealed that tribochemistry plays a major role in the significant wear rate reductions achieved in PTFE and alumina composites. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy reveal that PTFE chains break due to the mechanical stresses at the wear surface and, in the presence of oxygen and water in the ambient environment, produce carboxylic acid end groups. These carboxylic acid end groups can chelate to the exposed metal on the steel surface and nucleate the formation of the transfer film. The resulting thin and robust fluoropolymer transfer film protects the surface of the steel and changes the sliding interface from polymer on steel to polymer on polymer transfer film. These effects keep friction coefficients and wear rates low and stable. Ultimately, the real mechanisms responsible for the exceptional wear performance of these materials are all about the tribochemistry. 

Four nonhalogenated ionic liquids (ILs) based on the same phosphonium cation are investigated in terms of the anion suitability for enhancing the lubricity of a biodegradable oil. For all test conditions, typical for industrial machine components, the lubrication is shown to be governed by nonsacrificial films formed by the physisorption of ionic species on the tribo-surfaces. The anionic structure appears to have an important role in the formation of friction modifying films. The orthoborate ILs exhibit the formation of robust ionic boundary films, resulting in reduced friction and better wear protection. On the contrary, the surface adsorption of phosphinate and phosphate ILs appears to antagonistically disrupt the intrinsic lubrication properties of the biodegradable oil, resulting in high friction and wear. Through additional investigations, it is postulated that the higher dissociation of orthoborate ILs in the biodegradable oil allows the formation of hierarchical and electrostatically overscreened layer structures with long-range order, whereas the ILs with phosphate and phosphinate anions exhibit low dissociation in biodegradable oil, possibly due to the ion pairs being surrounded by a hydrocarbon halo, which presumably results in weak adsorption to form a mixed interfacial layer with no long-range order. © 2021 The Authors. 

Biotribology and tribocorrosion are often not included in numerical or computational modeling efforts to predict wear because of the apparent complexity in the geometry, the variability in removal rates, and the challenge associated with mixing time-dependent removal processes such as corrosion with cyclic material removal from wear. The lollipop is an accessible bio-tribocorrosion problem that is well known but underexplored scientifically as a tribocorrosion process. Stress-assisted dissolution was found to be the dominant tribocorrosion process driving material removal in this system. A model of material removal was described and approached by lumping the intrinsically time-dependent process with a mechanically driven process into a single cyclic volumetric material removal rate. This required the collection of self-reported wear data from 58 participants that were used in conjunction with statistical analysis of actual lollipop cross-sectional information. Thousands of repeated numerical simulations of material removal and shape evolution were conducted using a simple Monte Carlo process that varied the input parameters and geometries to match the measured variability. The resulting computations were analyzed to calculate both the average number of licks required to reach the Tootsie Roll® center of a Tootsie Roll® pop, as well as the expected variation thereof.

The influence of thickener and additive interactions on grease lubricating performance is examined. Polypropylene and lithium complex thickened (Li-complex) greases were tested both as neat greases and with a 2 wt% addition of ZDDP and/or MoDTC. A combination of ZDDP and MoDTC in the polypropylene grease provided the lowest friction with greater longevity compared to the Li-complex grease with the same additives, independent of sliding speed, contact pressure, temperature or type of sliding: continuous vs. reciprocating. The additive combination of ZDDP and MoDTC provided the best antiwear performance in both greases. Depending on the grease sample type, EDS revealed the presence of iron, zinc, phosphorous, sulfur, and molybdenum within the tribofilms.

Fluoropolymers have unique mechanical, chemical, and tribological properties (low friction coefficients) but their use as solid lubricants is inhibited by high wear rates (1-5×10-4 mm3/Nm). The addition of certain types of α-alumina has been shown to reduce the wear rate of PTFE by over three orders of magnitude, but due to its extremely high molecular weight PTFE cannot be screw injection molded. However, PFA, a perfluorinated copolymer of tetrafluoroethylene (TFE) and a perfluorinated alkylvinyl ether (PAVE), can be. Teflon® PFA 340 samples with various weight fractions of α-alumina (0%, 5%, 7.5%, 10%) were injection molded, and samples from each mold were wear tested against stainless steel (P=6.3 MPa, v=50.8 mm/s). Experiments showed that the friction behavior of the PFA 340-α alumina composite was very close to that of both unfilled PFA 340 and PTFE-α alumina composites. The wear rate of unfilled PFA 340 was 1.4×10-4 mm3/Nm, and dropped to 4.0×10-8 mm3/Nm for the PFA-α alumina composites. Just as in the case of PTFE-α alumina composites, these PFA composites generated brown-colored tribofilms on both the polymer and metal surfaces, which were indicative of tribochemical changes. ATR-IR and FTIR spectra of each surface showed evidence for the generation of perfluorinated carboxylate salts and waters of hydration. This spectral similarity between PTFE and PFA 340 samples shows that the same tribological mechanism found in PTFE-α alumina composites is responsible for ultralow wear in PFA-α alumina composites as well. 

Human tactile evaluations were combined with tactile friction measurements to quantify the perceptual experience of touching coated panels. Monosized beads of nine different polymer compositions were added to a soft-touch waterborne two-component PUR coating. Introducing beads of different composition affected tactile perception.

The aim of this study was to evaluate whether smooth surfaces varying in surface chemistry could be perceptually distinguished with the sense of touch. A set of ten glass surfaces was prepared which varied systematically in terms of the molecular composition of a thin coating of low topography. The contact angle, contact angle hysteresis, and surface energy were evaluated as objective physical parameters characterizing each coating. Additionally, the interaction forces between a human finger and the different coatings were quantified and compared in terms of tactile friction coefficients. The surfaces were evaluated psychophysically in terms of perceived similarities and were then ranked according to pleasantness. The participants could perceptually distinguish between surfaces varying in surface chemistry and a primary and secondary perceptual dimension were identified as sufficient to distinguish them. The primary dimension correlates with surface free energy, but both tactile friction and surface energy contribute to this dimension depending on whether the coatings are organic or inorganic. The secondary dimension could not be identified explicitly in terms of a physical quantity but is discussed in terms of recent developments in the literature. Coated glass is characterized by high friction coefficient upon interaction with a human finger as well as significant hysteresis in the stroking directions (lower applied load and higher friction in the backward stroke). Despite the complexity of the tribology, pleasantness can be clearly linked to it, where low friction (high contact angle) materials receive a higher ranking. © The Author(s) 2018.

Polytetrafluoroethylene (PTFE) is a solid lubricant known for its low friction coefficient and high wear rate. When filled with a low volume percent of alumina particles (5 wt%), its wear rate is decreased over four orders of magnitude. The development of a thin, uniform and well adhered transfer film during sliding is partially responsible for this decrease in wear rate by creating a low shear interface and forming a protective layer between the PTFE/alumina sample and metal countersample. In this work, a ''striped'' transfer film was generated by sliding up to one million reversals over a gradually decreasing stroke length. Wear and friction experiments were performed on a microtribometer to determine the robustness of the transfer film. Interferometry and profilometry were used to measure the height and wear of the film. Microscopy was used to investigate the morphology of the transfer film over sliding distance. 

One of the most dramatic reductions in the wear of PTFE has been achieved by compositing PTFE with as little as 1–5 wt% of alumina particles; this has been reported to produce wear rates K ∼10−7 mm3/Nm. The mechanisms associated with this reduction in wear are multifaceted, including 1) preventing crack propagation and delamination of the PTFE wear surface, 2) promoting tribochemistry and more recently 3) tribologically-induced breaking of the filler into nanoscale fragments to stabilize and reinforce tribofilms. However, in an effort to keep experiments comparable, many of the studies throughout the literature have focused on a narrow contact pressure range. In these experiments, we explored the effects of contact pressure on the tribological behavior of different PTFE and alumina composites, one of which is reported to achieve ultra-low wear (∼10−7 mm3/Nm) and another that is reported to only have mild reductions in wear (∼1 × 10−5 mm3/Nm) compared to unfilled PTFE (∼4 × 10−4 mm3/Nm). We found that with decreased contact pressures, the PTFE-alumina composite that was previously reported as high wear could achieve ultralow wear rates. The PTFE-alumina composite previously reported to achieve ultralow wear achieved ultralow wear at a range of low to high contact pressures, with a higher pressure limit corresponding to increases in wear. The friction behavior of PTFE-alumina composites was found to be highly dependent on contact pressure, with increasing pressures resulting in decreasing friction coefficients (∼0.5–0.17 over a 0.62–8.5 MPa range). This effect became more pronounced when the contact pressure was incrementally varied during testing resulting in up to a 70% decrease or increase in friction coefficient due to increasing or decreasing the pressure, respectively. IR spectra of the polymer wear surface showed that tribofilms rich in carboxylates and metal oxides form at the full range of contact pressures tested, even at the extremes. This formation of tribofilms at the sliding interface not only contributes to the ultralow wear of these materials, but plays a role in the friction behavior observed. From this, we gained new insight into the role, functionality and limitations of the alumina fillers. 

Calcium carbonate has attracted a lot of interest over the centuries. Nowadays, mainly as mineral pigment and filler, it has a wide use in technological applications ranging from paper, construction, polymers, and environmental solutions to consumer goods. Amongst these uses, the filler pigment is required to display either hydrophilicity (for applications in aqueous colloidal systems, including, for example, in paper and emulsion paints), or, in contrast, oleophilicity (for applications in contact with oil-based systems, such as plastics and volatile solvent-containing sealants).  To achieve oleophilicity, and resulting hydrophobicity, the filler is surface treated, typically using carboxylic fatty acids. In this thesis, effects of humidity and fatty carboxylic acids vapor on CaCO3 surface wettability and nanomechanical properties were studied, with the aim to gain knowledge on layer packing density and order, as well as resistance to water exposure and mechanical wear. A better understanding of the dynamic nature of the calcite surface presented in this work allows the industry to increase sustainable control over materials production and storage. First, a setup combining an atomic force microscope (AFM) with a humidifier was used to map nanomechanical properties of growing surface domains (hydrated form of CaCO3) formed by ion dissolution, diffusion, and redeposition, a process that is not reversible upon drying. Secondly, AFM and contact angle goniometer measurements showed that the stability of the calcite surface improves with increasing carboxylic acid chain length (C2 to C18). Meanwhile, X-ray photoelectron spectroscopy and vibrational sum frequency spectroscopy techniques demonstrated that a coherent layer with maximum packing density of carboxylate and carboxylic acid species was achieved with the use of stearic acid (C18) with high enough vapor pressure and exposure time. The AFM images successfully visualized that a complete C18 monolayer is capable of countering nano-wear of the calcite surface despite the humidity (under the range of loads investigated in this work) and the layer has self-healing properties, while calcite displayed high abrasive wear. Further, when calcite coated by a highly packed monolayer of C18 was covered with a water droplet, a large contact angle hysteresis resulted in a coffee ring effect (CRE). That is leading to formation of hillocks at the contact line consisting of dissolved fatty carboxylic acid and possibly calcium bicarbonate Ca(HCO3)2 molecules transported from the bare calcite region that also is created next to the droplet edge. Interestingly, C18 coated calcite remained considerably more stable in the case where a water droplet saturated with octanoic acid was used instead of water; thus, it was concluded that the CRE can be contained via reduction of the liquid surface tension and contact angle hysteresis.

Understanding the wear of mineral fillers is crucial for controlling industrial processes, and in the present work, we examine the wear resistance and nanomech. properties of bare calcite and stearic acid-modified calcite surfaces under dry and humid conditions at the nanoscale. Measurements under different loads allow us to probe the situation in the absence and presence of abrasive wear. The sliding motion is in general characterized by irregular stick-slip events that at higher loads lead to abrasion of the brittle calcite surface. Bare calcite is hydrophilic, and under humid conditions, a thin water layer is present on the surface. This water layer does not affect the friction force. However, it slightly decreases the wear depth and strongly influences the distribution of wear particles. In contrast, stearic acid-modified surfaces are hydrophobic. Nevertheless, humidity affects the wear characteristics by decreasing the binding strength of stearic acid at higher humidity. A complete monolayer coverage of calcite by stearic acid results in a significant reduction in wear but only a moderate reduction in friction forces at low humidity and no reduction at 75% relative humidity (RH). Thus, our data suggest that the wear reduction does not result from a lowering of the friction force but rather from an increased ductility of the surface region as offered by the stearic acid layer. An incomplete monolayer of stearic acid on the calcite surface provides no reduction in wear regardless of the RH investigated. Clearly, the wear properties of modified calcite surfaces depend crucially on the packing d. of the surface modifier and also on the air humidity.

A profound understanding of the properties of unmodified and saturated fatty acid-modified calcite surfaces is essential for elucidating their resistance and stability in the presence of water droplets. Additional insights can be obtained by also studying the effects of carboxylic acid-saturated aqueous solutions. We elucidate surface wettability, structure, and nanomechanical properties beneath and at the edge of a deposited droplet after its evaporation. When calcite was coated by a highly packed monolayer of stearic acid, a hydrophilic region was found at the three-phase contact line. In atomic force microscopy mapping, this region is characterized by low adhesion and a topographical hillock. The surface that previously was covered by the droplet demonstrated a patchy structure of about 6 nm height, implying stearic acid reorganization into a patchy bilayer-like structure. Our data suggest that during droplet reverse dispensing and droplet evaporation, pinning of the three-phase contact line leads to the transport of dissolved fatty carboxylic acid and possibly calcium bicarbonate Ca(HCO3)2 molecules to the contact line boundary. Compared to the surface of intrinsically hydrophobic materials, such as polystyrene, the changes in contact angle and base diameter during droplet evaporation on stearic acid-modified calcite are strikingly different. This difference is due to stearic acid reorganization on the surface and transport to the water-air interface of the droplet. An effect of the evaporating droplet is also observed on unmodified calcite due to dissolution and recrystallization of the calcite surface in the presence of water. In the case where a water droplet saturated with octanoic acid is used instead of water, the stearic acid-coated calcite remains considerably more stable. Our findings are discussed in terms of the coffee-ring effect. © 2021 The Authors.